1. Field of the Invention
The present invention relates to bias fabrics and methods and apparatus for making the same, and more particularly to fabrics comprising one or more plies of yarns oriented at a bias such as 45.degree. relative to the direction of elongation of the fabric.
2. History of the Prior Art
Most fabrics are formed from threads or yarns which are oriented along the long axis of the fabric and at right angles relative thereto. For certain applications, however, it is desired that some or all of the yarns be at other than 0.degree. or 90.degree. relative to the long axis of the fabric. For example, certain fabric tapes are used in the reinforcement of aircraft wings and other aircraft structural components. To optimize strength, it is necessary that the yarns forming such tapes be oriented on the bias or at angles such as 30.degree., 45.degree. or 60.degree. relative to the long axis of the tape.
Unfortunately, conventional weaving apparatus and techniques lend themselves to 0.degree. and 90.degree. yarn orientation rather than bias orientation. In a conventional loom, reciprocating shuttles traverse the width of the fabric so as to interweave transverse warp yarns with longitudinal fill yarns, resulting in a conventional fabric of 0.degree. and 90.degree. yarn orientation. Consequently, inefficient techniques such as those involving substantial amounts of hand labor must typically be employed to make a bias fabric. For example, one common method of making fiber reinforced plastic composites of bias fiber orientation for structural applications is to start with a unidirectional tape made up of straight yarns that are parallel and adjacent to each other and that are impregnated with a resin. The tape is cut into segments of uniform length with the cuts being 45.degree. to the long axis of the tape. Each segment is placed adjacent another segment with the original tape edge of each segment positioned next to the original tape edge of the adjoining segment. The newly formed 45.degree. oriented tape is used as is or combined with another length of 45.degree. tape with the yarns or fibers of the second tape 90.degree. relative to those of the first tape. The resulting tape can then be combined with layers of pre-impregnated conventional woven fabric or with layers of unidirectional tape depending upon the needs of the designer.
One problem in working with pre-impregnated unidirectional tape is the separation of yarns within the tape during the handling operations required to make a bias tape. Separations also may occur during the layup of the oriented tape or non-uniformly tensioned yarns during subsequent manufacturing operations. This separation results in a loss of desired mechanical properties. This problem may be overcome to a substantial degree by starting with a unidirectional woven fabric instead of unidirectional tape. The fabric is comprised of straight, parallel, adjacent warp yarns secured by an adequate but minimum number of fine denier fill yarns. For example, the warp yarns may be high modulus graphite. The fill yarns may be polyester which does not contribute to the structural properties of the laminate.
While oriented tapes offer means of producing structural composites with desirable properties, they are costly because of the number of steps involved to produce the tape. To make .+-.45.degree. bias tape, sufficient unidirectional fabric must be woven for the two plies of the tape. This double quantity of fabric must be pre-impregnated and then cut and laid up in two plies with the proper yarn orientation in each ply.
One technique which has been attempted involves weaving a unidirectional fabric having the desired reinforcing yarns in the filling direction, with a lightweight binding yarn in the warp direction. Fabric so woven is cut to length, then reoriented by securing one edge and advancing the other edge by the amount necessary to cause the fill yarn to lie at an angle of 45.degree. with respect to the warp yarn. This technique solves part but not all of the problem, since it provides fabric with reinforcing yarns oriented in just one way. It is therefore necessary to use two separate layers of fabric to accomplish the need for the 90.degree. disposition of the two layers to achieve a .+-.45.degree. fabric.
Other examples of prior art fabrics and methods and apparatus for making the same are provided by French Pat. No. 1,358,056, U.S. Pat. No. 2,842,472 of Hartstein, U.S. Pat. No. 3,321,348 of Rupp, British Patent Specification No. 1,126,530, U.S. Pat. No. 3,039,169 of Frickert, Jr. and U.S. Pat. No. 4,071,647 of McMullen. These patents illustrate various different schemes for making bias fabrics in which some or all of the yarns are at 45.degree. or in any event other than 0.degree. and 90.degree. with respect to the long axis of the fabric. However, such schemes for the most part do not produce fabrics of the type needed for composite reinforcing applications, and instead are more concerned with using the fabric as a reinforcement for other material to which the yarns are usually directly attached during the yarn laying process. Moreover, the methods and apparatus disclosed therein include still other disadvantages and inefficiencies in terms of ease and accuracy of producing a bias fabric. Such deficiencies and disadvantages result for the most part from the differences in emphasis and purpose of such art when compared with the typical reinforcing composite applications contemplated by the invention. Thus as noted, some of the patents listed above are concerned with laying loops of yarn or other fibrous material on another member to add bulk and strength thereto. Moreover, the apparatus shown in the patents is not capable of laying yarns in a density suited for typical applications of bias fabrics contemplated by the present invention. For example, the yarn carriers in most such arrangements are as much as 1-2" in diameter and as such are incapable of a yarn spacing of less than about 1-11/2".
Accordingly, it is an object of the invention to provide improved bias fabrics.
It is a further object of the invention to provide improved bias fabrics of the type well suited for reinforcing applications such as in the formation of fiber reinforced plastic composites for structural applications.
It is a still further object of the invention to provide improved methods and apparatus for making bias fabrics.